The simple bitrate math that lets ffmpeg hit an exact target file size

A developer on DEV Community has outlined a straightforward arithmetic method to compress video files to a precise size limit using ffmpeg, eliminating the need for repeated trial-and-error exports. The core formula treats file size as bitrate multiplied by duration, then works backwards: given a known duration and a target size, the required bitrate becomes a single division. The process accounts for real-world factors such as container overhead (roughly 2%) and a fixed audio bitrate, with the remainder allocated to video. The author implemented this logic in a Rust command-line tool called DeepShrink, but emphasizes that the underlying math is the key insight. The approach is particularly useful when uploading to platforms like Discord or bug trackers that enforce strict file size limits.